Researches and development have been conducted actively with a photograph printing device which serves as a so-called digital exposure device employing an image display device, such as a liquid crystal display. The photograph printing device of this type controls passing of light emitted from a light source through each pixel in the liquid crystal display by driving each pixel in accordance with image information, and irradiates the light having passed through each pixel on a printing paper, thereby printing out an image corresponding to the image information onto the printing paper.
Incidentally, resolution of a printed out image by such a digital exposure device corresponds to the number of pixels in the liquid crystal device. In order to improve resolution, for example, only the number of pixels in the liquid crystal display has to be increased. In this case, however, the liquid crystal display as a whole is undesirably upsized because of the increased number of pixels.
In recent years, a technique so-called "pixel displacement" is generally used in improving resolution of the printed out image. The pixel displacement is a technique, in which either the liquid crystal display or the printing paper is displaced relatively with respect to the other by, for example, half the pixel, and exposure is effected in each displacement. Consequently, although the number of the pixels in the liquid crystal display remains the same, resolution can be improved as if the number of pixels in the liquid crystal display were increased. Therefore, in this case, image quality can be upgraded by a less expensive device of a smaller size. The following will describe a photograph printing device which can effect the pixel displacement.
As shown in FIG. 16(a), the photograph printing device includes a lamp 51 serving as a light source, an image display device 52 composed of, for example, a liquid crystal display, a lens 53, rotatable BGR filters 54, and an XY stage 56, which are sequentially positioned in this order along the direction of light emitted from the lamp 51 on an optic axis linking the lamp 51 and XY stage 56. The XY stage 56 is furnished with a table, which has a main surface perpendicular with respect to the optic axis and is allowed to move in two directions perpendicular to each other within a single plane. According to the above arrangement, when a printing paper 55 is placed on the XY stage 56, the printing paper 55 can be moved in either of the two directions perpendicular to each other in the same plane.
With the above arrangement, passing of light through each pixel in the image display device 52 is controlled by driving each pixel with an image signal. Thus, light emitted from the lamp 51 is modulated at each pixel in the image display device 52 to reach the lens 53 and then focused. Light having passed through the lens 53 is irradiated to the printing paper 55 through one of the BGR filters 54, for example, the B (Blue) filter. When printing of a blue image is completed, the BGR filters 54 are rotated, and by sequentially setting the G (Green) filter and R (Red) filter at the position matching on the optic axis, a green image and a red image are printed out sequentially in the same manner as above, whereby a color image is printed out onto the printing paper 55. FIG. 17(a) shows pixels which form the color image printed out onto the printing paper 55 by the first exposure (exposure of each of BGR).
When the first print out of the color image is completed, the XY stage 56 is driven so as to displace the printing paper 55 in a horizontal direction by one pixel, for example. Then, the second print out of a color image is effected in the same manner as above. FIG. 17(b) shows pixels which form the color image printed onto the printing paper 55 up to the second exposure.
Thereafter, the XY stage 56 is driven so as to displace the printing paper 55 in a vertical direction by one pixel, and the third print out of a color image is effected in the same manner as above. Finally, the printing paper 55 is displaced back in the horizontal direction by one pixel, and the fourth print out of a color image is effected in the same manner as above. FIGS. 17(c) and 17(d) show pixels which form the color images printed onto the printing paper 55 up to the third exposure and fourth exposure, respectively.
Each pixel in the image display device 52 is supplied with a different image signal as a driving signal in every exposure (print out) from first to fourth.
By effecting the pixel displacement in the above manner, resolution of the image printed out onto the printing paper 55 can be improved two times both in the horizontal and vertical directions, thereby making it possible to upgrade the quality of the image printed out onto the printing paper 55.
The foregoing explained the arrangement, in which the image display device 52 was fixed and the printing paper 55 was displaced by one pixel with respect to the image display device 52 in each exposure. However, as shown in FIG. 16(b), the printing paper 55 may be fixed, and the image display device 52 may be placed on the XY stage 56 so as to be allowed to move in two directions perpendicular to each other in a single plane perpendicular with respect to the optic axis, so as to displace the image display device 52 by one pixel with respect to the printing paper 55. In this case, pixels as shown in FIGS. 17(a) through 17(d) can be obtained by the pixel displacement as well.
Incidentally, as shown in FIG. 16(a), with the photograph printing device of the above arrangement, that is, of the type that displaces the printing paper 55, there will be no problem when the printing paper 55 is a standard size paper (so-called sheet paper), such as a B5 or A4 (specified below) paper, and a postcard. However, in case that a paper roll made of a lengthy piece of paper with a particular width, namely, a roll of paper, is used as the printing paper 55, it is very difficult to displace the printing paper 55 because of its length. For example, even if transportation mechanism and holding mechanism for a roll of paper are improved so as to realize the displacement thereof, the arrangements of the transportation mechanism and holding mechanism becomes so complicated that it is well predicted that the costs of the device itself will be increased.
If it is arranged in such a manner that the printing paper 55 is displaced only in the exposure section, the printing paper 55 is folded or wrinkled, thereby deteriorating the quality of the printing paper 55. Moreover, if an image is printed out onto such a deteriorated printing paper 55, the display quality and reliability of the image are reduced. For the reasons discussed above, it is preferable not to adopt this arrangement as much as possible.
Incidentally, the B5 and A4 are the size of papers defined by JIS (Japanese Industrial Standards). More specifically, B5 is a paper size of 182 mm.times.257 mm, and A4 is a paper size of 210 mm.times.297 mm.
On the other hand, according to the arrangement of the photograph printing device of the type which displaces the image display device 52 as shown in FIG. 16(b), vibration of the XY stage 56 is conveyed to the image display device 52 each time the image display device 52 is displaced. As previously mentioned, the image display device 52 is composed of, for example, the liquid crystal display. However, the liquid crystal display includes many scanning lines and signal lines for driving the individual pixels, and these lines are so thin and fragile to vibration, impact, etc. For this reason, with the above arrangement, vibrations of the XY stage 56 give adverse effects, such as breaking of the lines, to the image display device 52, and therefore, the display quality of the image display device 52 is degraded, and so is the quality of the printed out image.